1. Field of the Invention
The present invention relates to a dry etching method which is especially applicable for etching refractory metals or their compounds to form a pattern having a high aspect ratio.
In semiconductor industry, dry etching is widely used in various steps for fabricating various kinds of semiconductor devices. Recently, refractory metals such as tungsten (W), molybdenum (Mo) and titanium (Ti) or their silicides are coming to be used as electrodes or wirings of the devices, as the requirements for low resistivity and high temperature durability of the wirings increases. Since the packing density of the devices in integrated circuits (IC) is increasing, fine patterning of the wiring layer became essential. And an accurate etching of refractory metal patterns having a high aspect ratio is becoming important in the semiconductor industry.
2. Description of the Related Art
There are a variety of etching methods used in the semiconductor industry, such as plasma discharge etching, reactive ion etching (RIE), magnetically enhanced RIE, electron cyclotron resonance (ECR) etching, and so on. These etching methods were mainly developed for etching semiconductor materials, aluminum, dielectric materials such as silicon dioxide or photoresist films, and they are used in various processes for fabricating semiconductor devices utilizing respective etching features. These etching methods can also be applied for etching the refractory metals and their compounds which have a very high melting point. But some problems arise when they are applied for forming fine patterns of a wiring layer which is essential in recent semiconductor devices.
FIG. 1 shows a cross sectional view of an exemplary wiring used in ordinary semiconductor devices. In the figure, reference numeral 1 designates a substrate in which various semiconductor devices such as transistors or diodes (not shown) are fabricated. Insulating layer 2 is made of silicon dioxide (SiO.sub.2) for example, which covers the substrate 1 and thus insulates the substrate 1 from the wirings formed on it. Over the insulating layer 2 is formed a wiring layer which is made from a refractory metal, tungsten for example. In some cases they are formed by a silicide or compound of the refractory metal. The wiring layer is etched to form a wiring 3.
In order to form the wiring 3, the entire surface of the wiring layer is coated with a SiO.sub.2 layer 4 which is etched to form a wiring pattern by photolithography. The reference numeral 5 designates a photoresist pattern used for the photolithographic etching. FIG. 1 shows a state when the photoresist pattern 5 (shown by a broken line) is removed. Using the SiO.sub.2 pattern 4 as a mask, the wiring layer is etched to form the wiring 3. In some cases, the photoresist pattern 5 is left unremoved, and used as a mask for etching the wiring pattern 3. In ordinary etching processes, the etching is done in both the vertical and lateral direction. So, an undercut 6 appears on both sides of the pattern. As a result the width of the wiring pattern 3 becomes narrower than the width of the mask. In order to accomplish a fine pattern etching, such side etching must be avoided, because it reduces the preciseness of the pattern or requires unwanted spaces for the wirings. These are all undesirable for recent fine patterned devices or high packing density circuits.
Various methods are proposed to overcome such undercutting. One is to perform the etching at a lower gas pressure, for example few m Torr, which is hundreds of times lower than ordinary etching pressure. In such a low pressure environment, the mean free path of the ions becomes long, and the ions hit the surface of the target along the electric lines of force which are perpendicular to the surface of the target. So, etching proceeds vertically to the surface of the target.
Another method is to increase the bias voltage applied to the target. A similar effect is obtained, and undercutting can be avoided for aluminum wirings. But when such methods are applied to refractory metals, the etching rate is decreased especially in the former method. While in the latter method, the layer beneath the etched layer is damaged and undesirable defects appear for fine patterned devices.
On the other hand, an etching method for high melting point metals such as W has been investigated, and it has been found that SF.sub.6 is effective in increasing the etching efficiency of W. Further detail can be seen in for example, Japanese Laid Open Pat. No. 61-48924, published on Mar. 10, 1986 "Dry Etching Method for High Melting Point Metals" by S. Adachi et al. But more or less undercut was inevitable for wirings made of refractory metals or compounds thereof.
Recently, considerable improvements have been made in the reduction of side etching in the dry etching process. It has been found that side etching is remarkably prevented if silicon tetrachloride (SiCl.sub.4) is used for etching silicon. This is considered due to the fact that, silicon is released from SiCl.sub.4 by the plasma, and the released silicon is deposited on the side wall of the etched groove and protects the side wall from the side etching. Further detail can be seen in "Trench Etching of Silicon by Hex Reactor" by G. K. Herb et al., Solid State Technology, December, 1987 (Edition translated in Japanese). But such an etching method can not be applied for refractory metals.
Further, an attempt has been proposed to use a masking material including SF.sub.6 to form a protection layer on the side wall of the etched groove formed in tungsten film to protect it from the side etching. Details of such a method is described in "ANISOTROPIC ETCHING OF TUNGSTEN" by T. Saito et al, pp. 240-249, Spring Meeting of ECS (Electro Chemical Society) 1988, held in Atlanta U.S.A. In this method, there is a danger that the released sulfur (S) might contaminate the surface of the device, and stain the etching apparatus.
These attempts are still under development, and there is no remarkable dry etching method which is applicable for refractory metals or their compounds, and which prevents side etching of grooves or undercutting of patterns beneath the mask.